Spoof detection for biometric validation

ABSTRACT

The invention provides an Optical Coherence Tomography (OCT) system capable of acquiring two orthogonally polarized depth scans from a target such as the fingerprint region of a finger. In the preferred embodiment the birefringence of tissue components and, optionally, other aspects of the target are measured in order determine a characteristic of the target, such as whether it is real of fake finger.

CROSS REFERENCES TO RELATED PATENTS OR APPLICATIONS

This US patent application, claims priority from provisional patentapplication 62/584,029 filed on 9 Nov. 2017. This US patent applicationis related to U.S. Pat. No. 9,721,138 titled “System and method forfingerprint validation, related to U.S. Pat. No. 7,526,329 titledMultiple Reference Non-invasive Analysis System, and U.S. Pat. No.7,751,862 titled Frequency Resolved Imaging System, all three of whichare incorporated by reference as if fully set down herein.

FIELD OF THE INVENTION

The invention described and illustrated in this application relates tothe field of non-invasive imaging and analysis and measurement oftargets such as tissue to image the tissue or to measure theconcentration of analytes. In particular the invention relates toimproving the performance of the non-invasive interferometrictechnologies such as Optical Coherence Tomography (OCT) for imaging andanalyzing tissue including, but not limited to, skin tissue and retinaltissue. Such analysis includes using OCT to determine the concentrationof analytes such as glucose in tissue or tissue fluids.

BACKGROUND OF THE INVENTION

The invention relates to non-invasive imaging and analysis techniquessuch as Optical Coherence Tomography (OCT). In particular it relatesusing optical interferometric techniques to monitor or measure surfaceand sub-surface attributes of human tissue to validate that the tissuebeing analyzed is living human tissue and is not synthetic or faketissue. The invention may be used in conjunction with other surfaceimaging techniques. Validating that the tissue under analysis is notsynthetic or fake is useful in secure identification, verification oflife, authentication of identity, and other bio-metric applications.

Non-invasive imaging and analysis of targets is a valuable technique foracquiring information about systems or targets without undesirable sideeffects, such as damaging the target or system being analyzed. In thecase of analyzing living entities, such as human tissue, undesirableside effects of invasive analysis include the risk of infection alongwith pain and discomfort associated with the invasive process. In thecase of quality control, it enables non-destructive imaging and analysison a routine basis.

Optical coherence tomography (OCT) is a technology for non-invasiveimaging and analysis. There are more than one OCT techniques. TimeDomain OCT (TD-OCT) typically uses a broadband optical source with ashort coherence length, such as a super-luminescent diode (SLD), toprobe and analyze or image a target. Multiple Reference OCT (MRO) is aversion of TD-OCT that uses multiple reference signals. Another OCTtechnique is Fourier Domain OCT (FD-OCT).

A version of Fourier Domain OCT, called Swept Source OCT (SS-OCT),typically uses a narrow band laser optical source whose frequency (orwavelength) is swept (or varied) over a broad wavelength range. InTD-OCT systems the bandwidth of the broadband optical source determinesthe depth resolution. In SS-OCT systems the wavelength range over whichthe optical source is swept determines the depth resolution.

Another version of Fourier Domain OCT, often referred to as SpectralDomain OCT (SD-OCT), typically uses a broad band optical source and aspectrometer to separate out wavelengths and detect signals at differentwavelengths by means of a multi-segment detector.

OCT depth scans can provide useful sub-surface information including,but not limited to: sub-surface images of regions of tissue; measurementof thickness of layers of tissue; magnitude of regions of abnormaltissue growth; measurement of concentration of metabolites, such asglucose, in tissue fluids; measurement of concentration of metabolites,such as glucose, in blood. More generally OCT depth scans can provideuseful sub-surface information regarding attributes of tissue.

The propagation of light through tissue and the scattering of light bytissue involve complex processes that alter the polarization state oflight. Plane polarized light waves are light waves that vibrate in asingle plane. A birefringent material causes the vibration plane of thelight waves to rotate.

Human tissue in the fingerprint region at the tip of the finger is acomplicated birefringent material. Different tissue layers havedifferent degrees of birefringence, and, because tissue is soanisotropic, the amount of rotation within these layers varies. Thosetissue layers having higher collagen content, such as the dermis, havehigher birefringence. Thus, the propagation of polarized light throughtissue undergoes complicated changes.

For example, as light propagates through tissue, the stratum corneum,the outer most layer of the epidermis, randomly rotates the state ofpolarization of light. As light continues through the tissue, it entersthe papillary dermis layer where the polarization substantially remainsin the arbitrary polarization state that was established in the stratumcorneum. Upon reaching the deep dermis region (the reticular layer), thepolarization state of light is further randomly rotated.

In addition to polarization modifying effects of real tissue, there arescattering coefficient related intensity variations in the in the lightscattered from different layers, such as the epidermis and dermis.Furthermore speckle size and distribution in the separate layers havedifferent characteristics.

It is often useful to acquire OCT sub-surface scans of tissue at knownlocations with respect to the tissue surface. While OCT can produce twodimensional images of the surface of a target such as tissue, there areconventional imaging technologies that can capture surface images, suchas a camera employing a conventional charged coupled device (CCD). Suchconventional imaging devices can readily capture images of the surfaceof tissue.

Tissue can be imaged to acquire a surface fingerprint by varioustechniques including, but not limited to: cameras using one or moreconventional charged coupled device (CCD); an array of conductingsensors in conjunction with an RF generator (as in an iPhone fingerprintdetector); ultrasonic imaging systems, such as those using capacitivemicro-machined ultrasound transducers (CMUTs) or similar piezo baseddevices (PMUTs).

While existing fingerprint sensors, such as those using ultrasoundtechniques or those using an array of conducting sensors in conjunctionwith an RF generator, are used to ensure use by authorized individuals,such sensors are vulnerable to being hacked, for example, by artificial(stick on) fingerprints or fake fingers. Synthetic materials are beingdesigned to more accurately mimic human tissue.

There is therefore an unmet need for a more reliable technique fordistinguishing between real living human tissue and synthetic or faketissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of Sheet 1 depicts an MRO version of OCT capable of measuring twoorthogonally polarized depth scans of a target.

FIG. 2 of Sheet 2 depicts two examples of B-scans of tissue.

FIG. 3 of Sheet 3 depicts a B-scan of a fake fingerprint on the surfaceof a real finger.

FIG. 4 of Sheet 4 depicts a preferred embodiment of the system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In the preferred embodiment, depicted in FIG. 1 Sheet 1, an MRO versionof OCT system capable of acquiring both polarization components of thelight back-scattered from the tissue under analysis is used to acquiredepth scans of the tissue to be analyzed.

In the preferred embodiment, circularly polarized light is applied tothe target which consists of either real or synthetic (fake) tissue,however in other embodiments linearly polarized light is applied to thetissue. For purposes of this application “real” tissue refers to “realliving” tissue.

The MRO system of FIG. 1 Sheet 1 has a broadband optical source such asa superluminescent diode (SLD) 101, controlled by an SLD controller 102that emits light 103 at a center wavelength of approximately 1300 nmwith a bandwidth of the order of 45 nm. The source is collimated by alens (L1) 105 which outputs collimated infra-red light. The light passesthrough a polarizer (POL1) 107 where it is converted into a linearlypolarized beam. It is transmitted through a half wave plate (HWP1) 109and a quarter wave plate (QWP1) 111 that transforms the linearlypolarized light beam into circularly polarized light, which is directedat an 80/20 non-polarized beam splitter (BS1) 113.

BS1 splits the light into a sample beam 115 and reference beam 117. BS1being a non-polarized beam splitter for has the advantage of applyingcircularly polarized light to the tissue target. Furthermore, bychoosing BS1 to transmit a higher proportion of light energy than isreflected (for example, 80% T and 20% R, or 80/20), a greater amount ofthe back-scattered light is transmitted to the detectors from thesample.

The sample beam 115 is then directed through a focusing lens (L2) 119 toa Galvo scanner 123 that directs the beam to a cylindrical lens (notshown) that provides the platen onto 121 which a finger to be analyzedis placed. The Galvo 123 performs 1D lateral scanning while the completesystem, with the exception of the cylindrical platen, is translated inthe orthogonal lateral direction to achieve 2D lateral scanning. Lightback scattered from the tissue target returns to the Galvo 123, throughL2 to BS1 where 80% is transmitted through to the detection system.

The reference path beam 117 passes through a second polarizer (POL2)125, an attenuator (Attn) 127 and a second quarter wave plate (QWP2) 129before being applied to a 50/50 non-polarizing beam splitter (BS2) 131.Note Attn 127 may be positioned before or after POL2 or QWP2. The lighttransmitted through BS2 passes through a focusing lens (13) 133 to thepartial mirror (PM) 135 in front of the reference mirror RM 137 on anoscillating voice coil. The multiple reflections between the RM and PMform the composite reference radiation that is the basis of generatingmultiple interference signals when combined with the light backscattered from the target. The light that is reflected at BS2 can beused to monitor the system light power as needed.

Upon reflection from the PM/RM, half of this composite reference lightis directed by BS2 to a polarized beam splitter (PBS1) 139. The otherhalf of the composite reference beam is eliminated by the QWP2 129, Attn127 and POL2 125 combination thus eliminating light feedback andinterference to the source and detection system. PBS1 139 splits thecomposite reference light into two orthogonal polarized beams. One ofthese components is reflected by the mirror (M1) 141 to the 50/50non-polarizing beam splitter (BS3) 143, while the other component isreflected by the mirror (M3) 145 to the 50/50 non-polarizing beamsplitter (BS4) 147.

The back-scattered sample signal collected by L2 119 is transmittedthrough BS1 113 to the polarized beam splitter (PBS2) 149, which splitsthe back-scattered sample light into two orthogonal polarizedcomponents. One of these polarization components of the back-scatteredsample signal is reflected to BS3 143 where it is combined with asimilarly polarized component of the composite reference signal to formtwo complementary interference signals. One of these interferencesignals is focused by a lens (L4) 149 onto a photo-detector (PD1) 157,while the second signal is re-directed by mirror (M2) 151 and focused bya lens (L5) 153 onto a photo-detector (PD2) 155. The combination ofthese sample and reference signals enable balanced detection of onecomponent of the back-scattered light from the tissue.

The second polarization component of the back-scattered sample signalthat is transmitted through PBS2 149 is combined with similarlypolarized component of the composite reference signal at the 50/50non-polarizing beam splitter (BS4) 147 to form two complementaryinterference signals. One signal is focused by a lens (L6) 159 onto aphoto-detector (PD3) 161 while the second signal is re-directed bymirror (M4) 163 and focused by a lens (L7) 165 onto a photo-detector(PD4) 167. This combination of sample and reference signals yieldsbalanced detection of the two orthogonal components of back-scatteredlight from the tissue.

The electronic signals output by PD1/PD2 in Detector 1 and PD3/PD4 inDetector 2 are combined in a balanced detection mode and applied to atrans-impedance amplifier prior to digitizing and digital signalprocessing.

In an alternative embodiment a quarter wave plate (QWP4) 169, shown withdashed lines surrounding it, is installed in the MRO system at theindicated location and changes the characteristics of the detectedsignal at each detector.

In both embodiments the differences between the signals from PD1/PD2 inDetector 1 and those of PD3/PD4 in Detector 2 are analyzed and used todetermine the manner in which the two orthogonal polarization componentsdiffer from each other as a function of depth.

FIG. 2 Sheet 2 depicts two examples of a set of adjacent depth scansacquired by the MRO system displayed as images. The data sets comprisingsuch images for the two orthogonal polarization components areprocessed, for example by subtraction, to determine differences betweenthe two images and hence information about the birefringence of tissueas a function of depth.

In some embodiments the birefringence of specific layers of tissue isanalyzed and compared with typical birefringence values of such specificlayers to determine if the target under analysis is real tissue or faketissue (i.e. a spoof).

In some embodiments the birefringence of specific interfaces of layersof tissue is analyzed and compared with typical birefringence values ofsuch specific layer interfaces to determine if the target under analysisis real tissue or fake tissue (i.e. a spoof).

In some embodiments the change in birefringence, due to an environmentchange, of specific layers or interfaces of layers of tissue is analyzedand compared with typical birefringence values of such specific layersor layer interfaces to determine if the target under analysis is realtissue or fake tissue (i.e. a spoof).

In some embodiments an environment change includes, but is not limitedto: a change in the pressure with which the finger to be analyzed isapplied to a platen. In some embodiments an environment change includes,but is not limited to: a change in the temperature of the finger to beanalyzed.

In some embodiments multiple adjacent B-scans are acquired to form avolume data set to be analyzed. In some embodiments a volume data set isacquired by employing lateral scanning techniques other than rasterscans or stepped B-scans.

One or more B-scans or a volume scan are analyzed to determineconsistency with either real tissue or synthetic tissue and therebyvalidate real tissue or detect a spoof comprised of fake synthetic orfake tissue.

Some embodiments verify that the level of birefringence of differentlayers is appropriate. Check if the average birefringence of differentlayers falls within a normal range for real tissue birefringence anddetermine a parameter or figure of merit that is a measure of thelikelihood of there being a spoof layer present.

Some embodiments verify that the level of birefringence of differentlayer interfaces is appropriate. Check if the average birefringence ofdifferent layers interfaces fall within a normal range for real tissuebirefringence and determine a parameter or figure of merit that is ameasure of the likelihood of there being a spoof layer present.

In addition to or instead of analysis of birefringence to determineconsistency with either real tissue or synthetic tissue, analysis ofsome or all of aspects described in the following embodiments may beincluded to determine a set of parameters or a set of figures of meritrepresentative of actual living tissue. Note, the term appropriate isused in the following steps to mean consistent with typical attributesof real tissue i.e. that they do not deviate significantly from therange of values found in normal human tissue.

Some embodiments verify that the appropriate number of layers arepresent in the fingerprint B-scan image (epidermis, dermis, etc.) withan appropriate relationship and that their are no additional layers thatwould correspond to a fake layer. For example, automatically segment theB-scan image into the two most optimal layers (regardless of actuallyhow many layers exist). Assume they are a superficial layer and a deeplayer. Compute the average thickness of the superficial layer and checkif the thickness is consistent with a normal epidermal thickness andthereby determine if there is a spoof layer present, or determine thelikelihood of there being a spoof layer present.

Alternatively, or in addition, blindly re-segment both the superficiallayer and the deep layer each into two more layers. Examine the new“superficial-superficial” boundary (located above the oldsuperficial-deep boundary) and the new “deep-deep” boundary (locatedbelow the old superficial deep boundary). Ensure that these new,putative boundaries do not correspond to actual, extra boundariesbetween extra layers (i.e. spoof layers) by comparing the mean verticalgradient (averaged over transverse position) of thesuperficial-superficial and deep-deep boundaries to a threshold. Computea figure of merit as a measure of the likelihood of there being a spooflayer present. Alternatively, or in addition, automatically segment theB-scan image into three layers and determine a parameter or figure ofmerit for the presence of three layer. The parameter or figure of meritis used as a measure of the likelihood of there being a spoof layerpresent. An example of a B-scan with an extra layer due to a transparentfake fingerprint is depicted in FIG. 3 Sheet 3.

Some embodiments verify that there are appropriate intensity variationswithin the epidermis and dermis layers, at their boundaries and thattheir relative intensities are appropriate. For example, compute theaverage intensity of the superficial layer and the average intensity ofthe deeper layer. Check if the average intensities fall within a normalrange for tissue scattering. A layer that is either too optically clearor too optically dense may not represent tissue. The ratio of theaverage intensities of the superficial layer and the deep layer shouldalso have a limited range in tissue. The average intensities and theirratio are used to determine a parameter or figure of merit that is ameasure of the likelihood of there being a spoof layer present.

Some embodiments verify that the speckle size and distribution in theseparate layers is appropriate. Compute the two-dimensional spatialFourier transform of the speckle profile in the superficial layer, andthe spatial Fourier transform of the speckle profile in the deep layer.Tissue will have a certain speckle size, which should appear as awell-defined peak on the two-dimensional spatial Fourier transform.Other scattering media (e.g. milk, glue, etc.) should have acharacteristically different speckle size. Determine a parameter orfigure of merit based on the spatial Fourier transform that is a measureof the likelihood of there being a spoof layer present.

Some embodiments verify that the dermal-epidermal junction has anappropriate contour. For example, compute the one-dimensional spatialFourier transform of the boundary between the superficial and deeplayers. This boundary should have “bumps” corresponding to rete ridgesat a certain range of spatial frequencies. A flat boundary such as glassor plastic will not exhibit a characteristic spatial frequency.Determine a parameter or figure of merit based on the one-dimensionalspatial Fourier transform that is a measure of the likelihood of therebeing a spoof layer present.

Some embodiments verify that the contour of the sub-dermal B-scancorrelates appropriately with the surface B-scan or the correspondingline on a surface fingerprint. For example, compute the one-dimensionalspatial Fourier transform of the boundary between the superficial anddeep layers and compare this with a similar function at the same regionof the surface fingerprint and determine a parameter or figure of meritbased on the correlation between the two spatial Fourier transforms thatis a measure of the likelihood of there being a spoof layer present.

Some embodiments generate a correlation map to determine temporaldifferences in tissue regions by comparing successively acquiredco-located A-scans or closely adjacent A-scans or successively acquiredco-located B-scans. The correlation mapping technique is described inthe paper titled “Feasibility of correlation mapping optical coherencetomography (cmOCT) for anti-spoof sub-surface fingerprinting” (J.Biophotonics 1-5 (2013)/DOI 10.1002/jbio.20120000231), a techniquetypically used to identify blood flow.

Some embodiments (a) Compute correlation map. (b) Mark pixel locationsthat have a certain level of decorrelation. (c) Regardless of whether ornot these pixel locations actually represent flow, compute both adepth-based and transverse-based histogram of decorrelation pixellocations. (d) Ensure that the histogram meets some pre-determinedshape, such as (i) decorrelation pixels should be present in deep, butnot superficial layer, (ii) decorrelation pixels should be clustered insuperficial part of superficial layer, (iii) distribution ofdecorrelation pixels should not be uniform over depth, (iv) distributionof decorrelation pixels should not be uniform over transverse dimension.Determine a parameter or figure of merit based on one or more processedhistograms that is a measure of the likelihood of there being a spooflayer present.

Some embodiments verify that the dynamic behavior of at least some ofthe above aspects behave in an appropriate manner when subjected tochanging environmental factors, such as pressure. For example, measurethe variation in layer thickness with pressure and check if the averagevariation with pressure falls within a normal range for real tissue anddetermine a figure of merit that is a measure of the likelihood of therebeing a spoof layer present. As another example, measure the variationin birefringence of one or more layers with pressure and check if theaverage variation with pressure falls within a normal range for realtissue and determine a parameter or figure of merit that is a measure ofthe likelihood of there being a spoof layer present.

In some embodiments, one or more parameters are used to define a closedboundary in n-dimensional space within which B-scans representative ofactual, living fingerprints would be distinguished from B-scans ofsynthetic or fake fingerprints. Actual parameter ranges corresponding toactual, living fingerprints would be refined with the analysis ofadditional sets of real and spoof images.

In some embodiments, these parameters are used as input to a machinelearning algorithm such as a “support vector machine” (SVM) in order todistinguish between real and synthetic or fake fingerprints.

In some embodiments, one or more figures of merit would be combined todetermine the likelihood of there being a spoof layer present.

In some embodiments, these figures of merit are used as input to amachine learning algorithm such as a “support vector machine” (SVM) inorder to distinguish between real and synthetic or fake fingerprints.

Many variations of a spoof detection or presentation detection attacksystem using some or all of the steps listed above are possible.

The preferred embodiment is further illustrated in and described withrespect to FIG. 4 of Sheet 4 where an Optical Coherence Tomography (OCT)system capable of simultaneously acquiring two orthogonally polarizeddepth scans from a target such as the fingerprint region of a finger isdepicted.

The optical probe beam 403 is applied to the target to be analyzed,which in the preferred embodiment is the fingerprint area of a real orfake finger 405. The finger is depicted as resting on an optional platen407 that is transparent to the probe beam.

The OCT system has a first and a second optical path for reference beams(described above in the discussion of FIG. 1 Sheet 1) that generate afirst and second orthogonally polarized reference beams, whereinsimultaneous interrogation of two orthogonally polarized components ofprobe beam back scattered from the target is enabled, such that a firstorthogonally polarized depth scan of the target and a secondorthogonally polarized depth scan of the target are generated.

A processing module 403 or processor receives first and secondorthogonally polarized depth scans 409 and 411, and wherein theprocessor filters, digitizes and digitally processes the depth scans anddetermines the difference between said first and said secondorthogonally polarized depth scans, and thereby provides a measure ofthe birefringence of the target or of components of the target and,optionally, other aspects of the target in order determine acharacteristic of the target, such as whether it is real of fake finger.

Signals that comprise the two orthogonal depth scans 409 and 411 areconnected to a processing module 413 where the signals 409 and 411 arefiltered, digitized and digitally processed to measure the birefringenceof tissue components and, optionally, other aspects of the target inorder determine a characteristic of the target, such as whether it isreal of fake finger.

In another preferred embodiment, an additional conventional imagingdevice (not shown) is used to acquire a surface image of the target tofacilitate determining the identity of the fingerprint and to furtherassist in distinguishing between a real and fake finger.

In another preferred embodiment, additional strain gauges, or otherpressure measuring devices, (not depicted) are embedded in the platen todetermine the pressure being applied by the finger on the platen andthereby enable dynamic changes in birefringence to be measured.

In another preferred embodiment, the platen 407 can deform or bedepressed proportional to the pressure being applied by the finger onthe platen and thereby enable dynamic changes in birefringence to bemeasured. Since the location of the surface of the target platenresponds to pressure of the target, such that change in location can bedetermined from a B-scan images (from the first and second orthogonallypolarized depth scans) that includes the surface of the platen, therebyenabling changes in birefringence of said target due to change inpressure is measured.

Many variations of the OCT system used to detect the interference. Whilein the preferred embodiment an MRO system is the OCT system capable ofdetecting two orthogonal polarization components independently isdepicted and described, in other embodiments other OCT systems could beused including, but not limited to, SS-OCT or FD-OCT or mode-locked OCTsystems.

In the preferred embodiment, the characteristic of the target to bedetermined is whether it is a real finger of a fake finger. In otherembodiments, the characteristic of the target to be determined is adifferent characteristic, such as the glucose concentration of a tissuecomponent.

In embodiments where birefringence is not being measured, simpler MRO orOCT systems could be used.

In some embodiments, the parameter set or figures of merit set used torepresent the tissue under analysis could be weighted by one or moreknown attributes of the subject whose identity the fingerprint purportedto correspond to. Such attributes include, but are not limited to,gender, race, age, physical weight and size, medical conditions, such asdiabetes.

The (weighted or un-weighted) parameter set or figures of merit setderived by analyzing one or more B-scans or a volume scan of the tissueunder analysis is processed by an algorithm that distinguishes aparameter set or figures of merit set of actual, living fingerprintsfrom that of synthetic or fake tissue.

Such a distinguishing algorithm may be a machine learning algorithm oran algorithm derived from systematic analysis of the characteristics ofreal tissue and of synthetic or fake tissue.

Many variations of the above embodiments are possible. The scope of thisinvention should be determined with reference to the description and thedrawings along with the full scope of equivalents as applied thereto.

What is claimed is:
 1. An optical coherence tomography system, saidsystem capable of simultaneously acquiring two orthogonally polarizeddepth scans from a target, said system comprising: an optical coherencetomography system; a first and a second optical path for reference beamsthat generate a first and second orthogonally polarized reference beams,wherein simultaneous interrogation of two orthogonally polarizedcomponents of probe beam back scattered from said target is enabled,such that a first orthogonally polarized depth scan of said target and asecond orthogonally polarized depth scan of said target are generated; aprocessor, wherein said processor receives said first and said secondorthogonally polarized depth scans, and wherein said processordetermines the difference between said first and said secondorthogonally polarized depth scans, and thereby providing a measure ofthe birefringence of said target, and determines if said target is fake.2. The system of claim 1, further including a target platen, and apressure measuring device in said target platen, such that change inbirefringence of said target due to change in pressure is measured,using information from said first and said second orthogonally polarizeddepth scans.
 3. The system of claim 1, further including a target platenthat responds to pressure of the target, such that change inbirefringence of said target due to change in pressure is measured,using information from said first and said second orthogonally polarizeddepth scans.